Circuit arrangement for telephone systems



ay 9, 1939. G W, SUTTON 2,157,927

CIRCUIT ARRANGEMENT FOR TELEPHONE SYSTEMS Filed Nov. 9, 1935 650/? f Wan/1M 50770 Patented May 9, 1939 UNITED STATES PATENT OFFIQE CIRCUIT ARRANGEMENT FOR TELEPHONE SYSTEMS Application November 9, 1935, Serial No. 49054 In Great Britain November 12, 1934 3 Claims. .(C1.'179135) The present invention relates to telephone systems, and more particularly to circuit arrangements for or involving the use of variable resistance transmitters or microphones of the differential type.

By a microphone of the differential type is meant one which comprises what are in effect two microphone elements, these elements being arranged so that during the operation of the 3 microphone the resistance of one element is increased as the resistance of the other is decreased, and vice versa. Such microphones are connected to give a push-pull effect and are usually of the kind in which a single moving electrode is sit- ]:"1 uated between two fixed electrodes, the two microphone elements being formed by the variable resistance path between the moving electrode and one fixed electrode and the variable resistance path between the moving electrode and the other fixed electrode.

With a differential microphone, it is important that the two halves of the transmission system corresponding to the two halves or microphone elements of the microphone should be maintained in balance under all working conditions, as a lack of balance results not only in a decreased efiiciency of conversion from sound waves to alternating currents but also in a diminution of that freedom from the production of even 30 harmonics and from trouble due to a noisy source of microphone current which should result from push-pull operation.

It is well known that unless suflicient resistance in excess of that of the microphone itself is included in a microphone feeding circuit, a slow and periodic rise and fall of the microphone res stance known as breathing is likely to occur. To prevent such breathing of a microphone, one or more anti-breathing resistances may be included in the feeding circuit. In the case of a microphone of the differential type, breathing may effect the two halves differently and the two halves may be subject to it to different degrees.

According to one feature of the present invention, circuit arrangements for or involving the use of a differential microphone are such that direct current is fed to each of the two halves or microphone elements of the microphone via one or more anti-breathing resistances individ- 50 ual to the half or element, and are also such that as regards alternating currents generated by the microphone the paths via anti-breathing resistances are shunted by a path or paths of lower impedance comprising one or more condensers. The provision of separate anti-breathing resistances for the two halves of the microphone tends to obviate the occurrence of an undue lack of balance between the two corresponding halves of the transmission system, while the fact that alternating currents generated by the microphone do not have to pass through the resistance prevents these resistances from causing an undue lowering of the efliciency of the conversion effected by the microphone.

The invention has in view inter alia the provision of a sensitive transmitter arrangement suitable for use in cases where it is desired to operate a loudspeaking receiver without the aid of an amplifier or where it is desired to render it unnecessary for the source of sound to be situated close to the transmitter.

The previously mentioned feature of the invention and various subordinate features are exemplified in the two specific embodiments of the invention which will now be described with reference to the. accompanying drawing. Fig. 1 of the drawing shows a direct call system and Fig. 2 shows a telephone operator's instrument circuit.

Referring firstly to Fig. 1, the direct call system shown is suitable for providing telephonic communication between offices in the same or adjacent buildings. The system is primarily intended for providing telephonic communication between a principal and his secretary in an adjacent oflifce, and will be described in the terms applicable to such use, it being assumed that the apparatus shown on the left of the dotted line is situated in the principals room and the apparatus shown on the right of the dotted line is situated in the secretarys ofilce.

The principal is provided with a loudspeaking receiver LS, a differential microphone MA, a calling key with two make contacts KA and KB, and a pilot lamp PL. The secretary is provided with an ordinary receiver RE, a differential microphone MB similar to the principals microphone, a switch-hook with a contact SW, and a calling buzzer BZ. The secretarys microphone and receiver may be mounted together to form a hand-microtelephone, it being in any case intended that the secretary shall speak directly into the microphone. The buzzer BZ is short-circuited by the switch-hook contact SW when the secretary removes the receiver or hand-microtelephone from the switch-hook. The principals microphone MA is situated on (say) the principals desk, and it is not intended that it should be addressed at close quarters. Both microphones are of the carbon granule type having a moving electrode situated between two fixed electrodes.

Also situated in the principals room are two transformers TA and TB, a pair of non-inductive anti-breathing resistances RA, and a pair of noninductive anti-breathing resistances RB. Each of resistances RA is shunted by a condenser CA, and each of resistances RB is shunted by a condenser CB. In so far as the important components of speech currents are concerned, the condensers should have impedances low compared with the impedances of the resistances they shunt. Since the circuit is so arranged that none of the condensers is subjected to reversals of voltage, the condensers can conveniently be of the so-called electrolytic type. The connection of the system to a source of direct current is made in the principals room. The direct current may be derived from a battery, from direct current supply mains, or from a rectifier fed from alternating current supply mains. Four wires extend from the principals room to the secretarys oflice.

Contact KA of the principals calling key when closed connects one pole of the direct current source to the moving electrode of the principals microphone MA in a circuit which passes over pilot lamp PL. One fixed electrode of this microphone is connected to the other pole of the direct current source in a circuit which includes a primary winding PA of transformer TA and a resistance RA shunted by a condenser CA, and the other fixed electrode is similarly connected over a similar winding on the same transformer and a similar resistance-condenser combination. The secondary winding SA of transformer TA is connected to the secretarys receiver RE over two of the four wires which extend from the principals room to the secretarys office, the receiver circuit normally including buzzer BZ owing to the fact that one of the two wires (hereinafter referred to as the common purpose wire) is also used to operate the buzzer and to feed current to the secretarys microphone MB. The two similar primary windings PA of transformer TA are low-resistance windings wound in such a manner that a push-pull effect is obtained.

Contact KB of the principals calling key when closed connects one pole of the direct current source to one end of the common purpose wire, the other end of this wire being connected via buzzer BZ to the moving electrode of the secretarys microphone MB. Each fixed electrode of this microphone is connected to the other pole of the direct current source in a circuit which includes one of the four wires extending between the secretarys ofiice and the principals room, one of two similar primary windings PB of transformer TB, and a resistance RB shunted by a condenser CB. The secondary winding SB of transformer TB is connected directly to the loudspeaking receiver LS. The two primary windings PB of transformer TB are lowresistance windings and are arranged to give a push-pull effect.

The principal operates the calling key when conversation with the secretary is desired, thus completing circuits for the microphones, pilot lamp PL, and calling buzzer BZ. As soon as the secretary lifts the receiver or hand-micro-telephone buzzer BZ is short circuited by switchhook contact SW and the circuits are in a condition for conversation to take place.

Typical values for resistances RA and RB and condensers CA and CB for a supply voltage of the order of 45 are 200 ohms and 200 ohms and 250 microfarads and 250 microfarads respectively.

Referring now to Fig. 2, the telephone operators instrument circuit shown comprises a differential microphone M, a transformer T with a pair of primary windings P and a pair of secondary windings S, a condenser C, a line-balancing resistance RS, a receiver RE, and a pair of non-inductive anti-breathing resistances R. Wires L are connected to the cord circuits controlled by the operator. The arrangements on the secondary side of transformer T are of a well known form and do not concern the present invention. It must be understood that the drawing is schematic in character, jack contacts and the like being omitted.

One pole of the source of microphone current is connected to the moving electrode of the microphone M, and each fixed electrode of the microphone is connected to the other pole of the source in a circuit which includes one of the two similar primary windings P of transformer T and one of the pair of anti-breathing resistances R. The two primary windings are low resistance windings wound so that a push-pull effect is obtained. Insofar as the important components of speech currents are concerned, condenser C provides a path of an impedance which is considerably lower than that through a resistance R. Typical values for resistances R and condenser C for a supply voltage of 50 are 250 ohms and 2 microfarads.

In this case the condenser C is subjected to reversals of voltage when speech currents are produced by the microphone, so that the condenser cannot be of the electrolytic type.

Resistances R and resistance RS may be wound on the core of transformer T.

I claim:-

1. In a telephone circuit arrangement, a differential microphone comprising a diaphragm and a vibration responsive element on each side thereof, a transformer having two primary windings, two resistor elements, each of said resistor elements connected at one end to a series circuit comprising one of said primary windings and one of said vibration elements, a direct current source connected to the other end of said resistor elements and to the microphone diaphragm, said resistor elements preventing breathing action from taking place in said vibration elements due to the passage of the current therethrough, and a separate condenser connected directly in shunt of each resistor element to permit the passage of alternating current generated by the operation of said microphone, said condensers having a lower impedance than that of said resistor elements.

2.-In a telephone system, the combination of two telephone stations, a differential transmitter and a receiver for each station, two line circuits extending between said stations, a separate transformer having two primary windings and a secondary winding in each of said line circuits, the receiver of each station connected to a secondary transformer winding conductively isolated from associated primary windings and circuits thereof, each half of the differential microphone at a station connected to the associated two primary transformer windings, a separate resistance and a separate capacitance element also connected to each primary transformer Winding, and signalling means at each station connected to the mid-point of each of said differential microphones.

3. In a telephone circuit arrangement, a differential microphone comprising a diaphragm associated through a common electrode with each of two variable resistance elements, im-

5 pedance bridges each comprising a capacitance and. a resistance, a coupling transformer having two primary windings and a secondary winding, and a loudspeaker, a circuit for each variable resisance element of the difierential microphone 10 including a primary winding, an impedance bridge and a source of direct potential, another circuit conductively isolated from said circuits and'including said loudspeaker and said secondary winding, said impedance bridges although highly conductive to audible frequency currents, effective in reducing breathing in the variable resistance elements by limiting the direct current flow therethrough. 

